The commercial field of ceramic decalcomanias and their production and application has developed quite slowly over the years. Thus, while significant techniques in the production of various types of decals, including overglaze ceramic decalcomanias, have developed significantly over the years, many of the same techniques have now been used commercially for a number of years without significant modification or improvement.
One of these techniques relates to the use of wet printing for designs in ceramic decalcomanias, in which a design layer is wet printed onto a backing sheet, and in a most preferable embodiment known as the four-color technique, in which four different basic colors are applied sequentially in such wet ink formulations.
The decals themselves usually thus comprise multi-layered structures, which generally include a backing sheet, a design layer, and in most cases a protective layer which is applied on top of the design layer. The colors in the design layer are thus formed from inorganic pigments or oxides, and other layers can be used such as layers which facilitate release of the backing from the design layer and the like.
One of the various types of decals which are used in the ceramic industry is known as the "underglaze decal. " This type of decal is applied to the ware and itself before glazing. However, a far more useful type of decal which has been developed is the so-called "overglaze decal" which can be applied to the ware after high temperature glaze has been applied to the ware and fired. These overglaze decals have generally been of two classes, namely silk screen and lithographic decals. In silk screening processes a silk screen template or stencil is placed over the surface to which the pigment or color is to be deposited, and it is then applied through the screen.
In many prior art patents including British Pat. No. 1,094,104 to Johnson, Mathey & Co., ceramic pigment transfers are disclosed which include inks with a printing medium or varnish incorporating a ceramic pigment being applied to a backing sheet over which a covering layer of an adhesion promoting flux of glass forming constituents is applied. This covering layer can be fused to form a protective layer after firing, and the flux itself can include constituents which are adapted to form a lead borosilicate glass, such as lead oxide, boric acid and silica. However, in employing such techniques, since these glass forming constituents have different melting points, the application of same over a pigment causes the pigment to become spotty.
In accordance with U.S. Pat. No. 3,898,362, which is assigned to the assignee of the present application, an improved wet printing technique is disclosed in which overglaze ceramic decalcomanias are provided from a wet ink formulation free of glass and including oxide coloring agents and a liquid printing medium such as drying oil, varnish or resin. The decalcomanias are thus produced by wet printing the wet ink formulation onto a decalcomania backing sheet to form a wet design layer free of glass, and by then separately depositing onto the wet design layer a protective coating in the form of a prefused glass flux, which may also be initially deposited on the backing sheet and the wet design layer printed thereover. In this manner when the decalcomania is positioned on a ware and fired, the protective coating fuses and tightly binds the design layer to the ware.
The ink used in this technique can contain from about 30 to about 60 wt. % of the oxide colorant, and there is a general reference in column 5 of this patent to the possibility of printing a prefused glass flux as a direct or moist coating onto the backing paper prior to printing. This latter technique, however, has not found any commercial application, although the general technique disclosed in this patent has. The search for improved wet printing techniques, in which four-color design layers can be readily and accurately applied to such overglaze decalcomanias, has therefore continued.